1. Field of the Invention
The present invention relates to an acoustic wave device, a filter and a duplexer, and more particularly, to an acoustic wave device having a piezoelectric substance sandwiched between a pair of electrodes, a filter and a duplexer using such an acoustic wave device.
2. Description of the Related Art
There has been an increasing demand for light and compact resonators and filters using such resonators as wireless devices, which may be typically mobile communication equipment, have been spread rapidly. In the part, filters having surface acoustic wave (SAW) devices were mainly used. Recently, an attention to an acoustic wave device having a piezoelectric substance sandwiched between a pair of electrodes has been drawn because the acoustic wave device of this type has good performance at high frequencies in the order of GHz and can be miniaturized monolithically. Examples of the above-mentioned acoustic wave device are piezoelectric thin-film resonators such as an FBAR (Film Balk Acoustic Resonator) and an SMR (Solidly Mounted Resonator).
A filter structure as shown in FIG. 1 is frequently used in which resonators are arranged in series and parallel so as to form a ladder. More specifically, series resonators S1 through S3 are arranged in series between an input terminal Tin and an output terminal Tout. Parallel resonators P1 and P2 are connected between nodes between the adjacent series resonators and ground. The ladder type filter thus configured has advantages that the insertion loss and the degree of out-of-band suppression can easily be adjusted by changing the number of stages and the capacitance ratio of resonators arranged in series and parallel. It is therefore easy to design the insertion loss and the degree of out-of-band suppression.
The following document discloses a ladder type filter using FBARs in which two resonators are arranged in series between adjacent nodes or between an node and a terminal: 2002 IEEE ULTRASONICS SYMPOSIUM, pp. 907-910.
As is known, IMD (Inter Modulation Distortion) or CMD (Cross Modulation Distortion) takes place in the filters used in mobile communications or high-frequency wireless communications. Particularly, IMD or CMD is serious in devices that handle high power, such as duplexers. The IMD and CMD depend on non-linearity resulting from the device (resonator) structure and material. When signals of different frequencies are applied to the filter or duplexer, new unwanted frequency components will be generated in addition to the normal frequency response of the applied signals. The new frequency components may serve as noise in the receive filter.
FIG. 2 shows a variation in which the series resonator S1 shown in FIG. 1 is replaced by two resonators S1a and S1b connected in series. In this manner, each of the resonators between the adjacent nodes (S1-S3 in FIG. 1) and those between the nodes and ground (P1 and P2) may be divided into multiple resonators (S1a and S1b in FIG. 2). The use of divided resonators contributes improvements in power durability and IMD and CMD in high-power transmission. However, in practice, a secondary distortion still remains.